When a transform depends on the layout size of an element, one can see
visual distortions caused by the difference between the unsnapped size
used in the transform, and the snapped size calculated during scene
building. Ideally we could compute the transform after we snap, rather
than before. This patch adds support for a computed reference frame
which takes parameters to calculate the ideal transform dynamically.
In a future patch, we should make videos take advantage of this same
mechanism to avoid similar problems. This requires support for mirroring
and rotations.
Differential Revision: https://phabricator.services.mozilla.com/D77956
8f69882854